1. Technical Field
The present invention relates generally to a thermal contact arrangement between a processor and a heat sink, and more particularly to a processor thermal contact having at least a first and a second surface finish of differing smoothness.
2. Background of the Invention
Processors (also referred to as “computer processors” or “processor chips”) are specialized electronic circuits providing computing functionality in a variety of modern electronics, such as computers or other computing devices, networking devices, and/or telecommunications devices. Processors (“chips”) may be responsible for the overall operation of a computing, telecommunications, or network device (such as a central processing unit, router or switch), operation or coordination of a device's subsystem (such as a graphics or sound processor), particular operations (such as a math coprocessor), and so forth. During operation, processors generate heat as a result of their operation. The processor may be attached to a carrier such as a circuit board, as shown in the prior art view of FIG. 1.
Generally speaking, excessive temperature may disrupt a processor's operation or, in more severe cases, damage the processor. Accordingly and as shown in FIG. 1, a heat sink may be affixed to the processor in order to dissipate thermal energy generated by the processor. Similarly, heat sinks may be attached to other computing elements that generate heat in order to transfer heat away therefrom and safely dissipate the heat. A heat sink is one example of a thermal conductor, which may then dissipate the heat to the air, a liquid, or other similar cooling sub-system
The interface between the processor and heat sink may be referred to as a “thermal joint.” The rate of conductive heat transfer, Q, across the interface may be further refined to include the effects of contact resistance which then can be approximated by
  Q  =            KA      ⁡              (                  Tc          -          Ts                )              L  
where K is the thermal conductivity of an interface material (whether a dedicated thermal interface material discussed below, air, or another material), A is the heat transfer area, L is the interface thickness and Tc and Ts are the chip surface and heat sink temperatures. The thermal resistance of a thermal joint, Rc-s, is given by
      Rc    ⁢          -        ⁢    s    =            (              Tc        -        Ts            )        Q  
and on rearrangement,
      Rc    ⁢          -        ⁢    s    =      L    KA  
Thus, the thermal resistance of the thermal joint is directly proportional to the thermal joint thickness and inversely proportional to the thermal conductivity of the medium making up the thermal joint and to the size of the heat transfer area. Thermal resistance may be minimized by making the thermal joint as thin as possible, increasing thermal joint thermal conductivity by eliminating interstitial air and making certain that both surfaces are in intimate contact. The thermal resistance of the thermal contact arrangement (which, in one example, includes the thermal joint, processor or chip, and heat sink) may be generally expressed as the thermal resistance of the thermal joint plus the thermal interface resistances of the chip and heat sink:
  Rtotal  =            L      KA        +          Rc      ⁢              -            ⁢      i        +          Rsi      ⁢              -            ⁢      c      
where Rtotal is the total resistance of the thermal contact arrangement, Rc-i is the thermal resistance between the chip and interface material and Ri-s is the thermal resistance between the interface material and the heat sink.
FIG. 2 is a cross-sectional diagram taken along line 2-2 of FIG. 1. As shown in FIG. 2, a thermal interface material (TIM) may be sandwiched or placed between the processor and the heat sink. The TIM may facilitate or enhance heat transfer between the processor and heat sink, thus potentially reducing the temperature experienced by the processor and/or extending the processor life. The TIM essentially performs the functions of eliminating at least some interstitial air pockets and enhancing contact between the processor and heat sink. Further, a TIM typically has a high thermal conductivity K than air, and thus enhances the rate of conductive heat transfer Q.
TIMs, however, may suffer from migration over time. Put simply, some TIMs tend to move away from the thermal joint with time, flowing or otherwise migrating out from the heat transfer surface area of the processor and/or heat sink. As the TIM migrates, air pockets may form in the thermal joint, and rate of conductive heat transfer between processor and heat sink may drop. Thus, as time passes, the aforementioned problems may occur even though a TIM is initially used.